Concerning Ants and Altruism

This article was originally written for the Wellcome Trust science writing competition 2013. Do get in touch if you would like to feature this article, or something similar, in your own publication.

There is an ant whose only purpose in life is to eat. It’s force-fed by its fellow Myrmecocystus mimicus until its abdomen swells to the size of a grape. This unfortunate “honeypot ant” serves a grim purpose. When famine hits the nest, all the other ants will turn on it and rip it to shreds, devouring it for vital nutrients.

And honeypot ants aren’t the only ones. Campanotus truncatus soldier ants have heads shaped like wedges, to block nest entrances when the colony is under attack. Campanotus saudersi colonies deploy bomb ants with swollen abdomens full of poison and glue, ready to explode and sacrifice their lives to immobilise enemies. There are seed-miller ants with giant flour-grinding mandibles and minuscule fungi-farmer ants one hundredth of the size of the workers. All of them will work together like single, specialised cells in the body of one enormous organism, or die trying.

But if Darwin’s theory of evolution has taught us anything, it’s to preserve your own genes and pass them on to the next generation. So why aren’t the worker ants doing this?

Well, there are many ways to skin a cat, as the old adage goes. You don’t have to sire your own children to ensure the family line is continued. You can preserve your own genes by helping those you share them with – namely, your family. I know the prospect can seem distasteful, but helping your relatives, through a form of Natural Selection called Kin Selection, really is the foundation on which most animal societies are built.

W. D. Hamilton, the eminent 20th century Biologist, quipped: “I would lay down my life for two brothers or eight cousins”. What he meant by this morbid speculation was that the level of sacrifice you’d make for a relative is directly proportional to how closely related you are. If you share enough genes, altruism can be worth it. This was illustrated in 1964, with the aptly named Hamilton’s Rule:

rB > C

r represents how closely related the altruist is to the recipient. B represents how much the altruist’s action would benefit the recipient – more food, security, anything that would help them live longer and have more offspring. C represents the how much the action would cost the altruist, in terms of time, energy or safety. If the recipient’s relatedness to you and the benefits they gained outweighed the cost to yourself, it’d be in your interest to help them. If, however, the cost is too great, you’d be better off leaving all eight of your cousins drowning in the river and going off to raise your own family.

And that explains why ants cooperate with such clockwork precision. All workers are sisters and share at least 50% of their genes with their siblings. But that still leaves one puzzle – most colonies have one queen and up to 700,000 sterile workers. In fact, the big mama Army Ant Eciton burchelli can lay 200,000 eggs in two weeks. Workers may share 50% of their genes with their sisters, but surely they’d also share 50% of their genes with any offspring they might have. According to Hamilton’s Rule, they’d be just as well off starting families themselves. So why don’t they?

Unlike most sisters, ants don’t actually share 50% of genes. Something a lot more cunning is at work here – something called haplodiploidy. And no, that’s not a type of dinosaur.

Most animals are diploid. This means they have two sets of chromosomes – one inherited from their mother, and one from their father. Female ants follow this rule. But the few males in the nest are haploid – they develop from unfertilised eggs, so they only have one set of chromosomes, inherited from their mother. As long as the queen mates with only one male, which she usually does, all sisters will already have inherited 100% of their father’s chromosomes. Added to the 50% they share with their mother, this means workers share 75% of their genes in common (which would explain why they cooperate far better than many sisters I can think of). If they shacked up with a male, they’d only share 50% of their genes with their children.

In this light, how do human societies function at all? Apart from our family, most people we interact with nowadays aren’t related to us whatsoever, but we still offer them our seats on the bus. We’ll probably never see them again, so we’re not even hoping they’ll return the favour. But humans, thankfully, have far more complex and empathic brains than ants. One theory speculates that we help people we’ll never see again because it contributes to an overall society of cooperation, from which everyone benefits.